Contemporary magnetic hard disk drives typically include a rotating rigid storage disk and a bead positioner for positioning a data transducer at different radial locations relative to the axis of rotation of the disk, thereby defining numerous concentric data storage tracks on each recording surface of the disk. The head positioner is typically referred to as an actuator. Although numerous actuator structures are known in the art, in-line rotary voice coil actuators are now most frequently employed due to their simplicity, high performance, and their ability to be mass-balanced about their axis of rotation, the latter being important for making the actuator less sensitive to perturbations. A closed-loop servo system within the disk drive is conventionally employed to operate the voice coil actuator and thereby position the heads with respect to the disk storage surface.
The read/write transducer, which may be of a single or dual element design, is typically deposited upon (or carried by) a ceramic slider structure having an air bearing surface for supporting the transducer at a small distance away from the surface of the moving medium. Single write/read element designs typically require two-wire connections while dual designs having separate reader and writer elements require two pairs of two-wire connections. Magnetoresistive (MR) heads having separate inductive write elements in particular generally require four wires. The combination of an air bearing slider and a read/write transducer is also known as a read/write head or a magnetic recording head.
Sliders are generally mounted to a gimbaled flexure structure attached to the distal end of a suspension's load beam structure. A spring biases the load beam and the head towards the disk, while the air pressure beneath the head developed by disk rotation relative to the slider pushes the head away from the disk. The gimbal enables the slider to present a "flying" attitude toward the disk surface and follow its topology. An equilibrium distance defines an "air bearing" and determines the "flying height" of the head. By utilizing an air bearing to support the head away from the disk surface, the head operates in a hydrodynamically lubricated regime at the head/disk interface rather than in a boundary lubricated regime. The air bearing maintains a spacing between the transducer and the medium which reduces transducer efficiency. However, the avoidance of direct contact vastly improves the reliability and useful life of the head and disk components.
Currently, nominal flying heights are on the order of 0.5 to 2 microinches. The magnetic storage density increases as the head approaches the storage surface of the disk. Thus, a very low flying height is traded against device reliability over a reasonable service life of the disk drive. At the same time, data transfer rates to and from the storage surface are increasing; and, data rates approaching 400 megabits per second are within practical contemplation.
The disk drive industry has been progressively decreasing the size and mass of the slider structures in order to reduce the moving mass of the actuator assembly and to permit closer operation of the transducer to the disk surface, the former giving rise to improved seek performance and the latter giving rise to improved transducer efficiency that can then be traded for higher areal density. The size (and therefore mass) of a slider is usually characterized with reference to a so-called standard 100% slider ("minislider"). The terms 70%, 50%, and 30% slider ("microslider", "nanoslider", and "picoslider", respectively) therefore refer to more recent low mass sliders that have linear dimensions that are scaled by the applicable percentage relative to the linear dimensions of a standard minislider. Sliders smaller than the 30% picoslider, such as a 20% "femtoslider", are presently being considered and are in early development by head vendors. As slider structures become smaller, they generally require more compliant gimbals; hence, the intrinsic stiffness of the conductor wires attached to the slider can give rise to a significant undesired mechanical bias effect.
Trace interconnect arrays typically support or aid in supporting the slider next to the data storage surface, and to connect read and write elements of the head with external circuitry. Two conductor paths are typically required for the write element, and two conductor paths are required for the read element, of the magnetic head. The interconnect array, typically formed on a polyimide film substrate, may extend from the slider to a preamplifier/write driver circuit, either directly, or via one or more intermediate interconnect trace arrays. These designs typically include trace segments extending from the flexure to a signal connection point which may be located on the side of the rotary actuator, for example. Since these conductor trace interconnect arrays are low in profile, and are precisely formed printed circuits upon plastic film substrates, they tend to have more predictable mechanical properties than discrete wire conductors used in the past, thereby improving tolerances in manufacturing and operation.
In transmission lines and interconnects of the type under contemplation, it is important to reduce the effect of the interconnect on the source (the preamp circuit for the read element and the write driver circuit for the write element, in a magnetic recording head, for example). The inductance and capacitance parameters of the trace array introduce a phase-change in the current/voltage waveforms, and most designs are made to minimize undesired effects of inductance and/or capacitance upon overall circuit performance. Moreover it is desirable to have a uniform characteristic impedance at any point along the trace array because such uniform characteristic impedance reduces signal distortion as well as minimizes reflections between the source and the head. The characteristic impedance per unit length of the traces included within the array is defined, at high frequencies, as the square-root of the ratio of the inductance to the capacitance of the traces.
One typical length of a trace array employed within a 3.5 inch disk drive, for example, is 45 millimeters (mm). One segment of this trace array is positioned over and in a close or contact relationship with the flexure and loadbeam structures and is approximately 18 mm in length. Another segment of the trace is supported in air and extends from the loadbeam to the preamplifier/write driver circuit and is approximately 27 mm in length. Other form factor disk drives, such as 2 inch disk drives or 3 inch disk drives for example, will also have similarly proportioned trace array segment lengths.
The 27 mm segment of the array is suspended in air because it is desirable during the manufacturing process to provide a flexible segment of the trace array to facilitate connection with the preamplifier/write driver circuit. As a result, the capacitance of the conductor traces in this segment of the array is relatively low with respect to ground. This relatively low capacitance results in a generally higher characteristic impedance per unit length in this segment of the array. On the other hand, the 18 mm segment of the array, which is positioned in close proximity to the flexure and loadbeam structures, which structures are typically formed of stainless steel, forms a ground plane between flexure and loadbeam structures and the 18 mm segment of the array. As a result, a capacitive coupling relationship is formed between this segment of the array and the stainless steel flexure/loadbeam structures. The capacitive coupling relationship results in an increase in the capacitance of the signal traces defined within this 18 mm segment of the array as compared to traces defined in the 27 mm segment of the array. This increase in capacitance results in a decrease in the characteristic impedance per unit length of the 18 mm segment of the array. Accordingly, the characteristic impedance per unit length of the 18 mm segment of the array is lower than the characteristic impedance per unit length of the 27 mm segment of the array. As previously stated, such characteristic impedance disparities along the trace array undesirably result in increased signal distortion as well as increased reflections between the slider and source.
Conventional trace interconnect arrays have a further capacitance disparity, which further contributes to the aformentioned characteristic impedance disparity, because the conductor traces defined within the array have varying geometries and spacings between each other as the traces extend from the slider to the preamplifier/write driver circuit. By way of example, the 18 mm segment of the trace array includes four conductor traces. The four conductor traces are arranged into two groups, where each group has two traces that extend along the longitudinal edges of the loadbeam between the slider and a region away from the slider. Each group of traces has a spacing arrangement: conductor trace, space, conductor trace of 40-30-40 microns respectively and a trace height of approximately 10 microns. Continuing to follow the traces back towards the 27 mm segment of the array, each trace within the two groups continues to taper outwardly to a final spacing arrangement: conductor trace, space, conductor trace of 100-30-100 microns wide respectively. Moving from the 18 mm segment of the array to the 27 mm segment of the array, the traces yet further continue to taper outwardly to a uniform spacing arrangement: conductor trace, space, conductor trace of 200-30-200 microns wide respectively throughout the 27 mm segment of the trace array and where the height of the traces in this region of the array are approximately 18 microns.
One method to reduce inductance and/or capacitance is to ensure that reactive components of the interconnect are minimal. There is usually a trade-off between the inductance and the capacitance, as reducing inductance by moving the conductors closer together increases the inter-conductor capacitance. Once conductor trace spacing and geometry is fixed at a minimum distance and shape, respectively, which is limited by manufacturing tolerances, the inductance and capacitance remains substantially constant at a particular frequency.
Since the characteristic impedance per unit length of the 27 mm segment is higher than the 18 mm segment as described above, it is desirable to hold the impedance of the 18 mm segment constant while modifying the impedance of the 27 mm segment to impedance match the 18 mm segment. One method of matching the characteristic impedance of the 18 mm and 27 mm segments of the array is to introduce a ground plane, such as stainless steel or metal foil, to the 27 mm segment of the array. The ground plane will provide a capacitive coupling relationship between the 27 mm segment of the array and the plane, which increases the capacitance of this segment of the trace. As a result, the characteristic impedance of this segment of the array is decreased to more closely match that of the 18 mm segment of the array. However, introduction of this ground plane arrangement to the 27 mm segment of the trace raises the cost of a relatively inexpensive trace array as well as introduces additional process steps to the manufacturing process of the array. Moreover, the addition of the ground plane will undesirably affect the flexibility of the 27 mm segment of the trace array, where such flexibility is necessary for bending and shaping the array during connection to the preamplifier/write driver circuit.
Thus, a hitherto unsolved need has remained for a trace interconnect array having more effectively controlled inductance and capacitance characteristics that does not introduce additional process steps or cost to the manufacturing process of the array.